The present invention is generally directed to toner processes, and more specifically, to the preparation of resin or polymer particles by a process comprised of (a) pre-emulsification and (b) emulsion polymerization, and which resin or polymer product particles can be selected for use in toner aggregation and coalescence processes, reference, for example, U.S. Pat. Nos. 5,278,020; 5,344,738; 5,403,693; 5,418,108; and 5,364,729, the disclosures of which are totally incorporated herein by reference. In embodiments, the present invention is directed to the economical in situ chemical preparation of toners without the utilization of the known pulverization and/or classification methods, and wherein toners with an average volume diameter of from about 1 to about 25, and preferably from 1 to about 10 microns, and narrow size distribution can be obtained, the size distribution as measured by GSD being in the range, for example, of about 1.05 to about 1.40, and preferably in the range of 1.05 to 1.3. The resulting toners can be selected for known electrophotographic imaging and printing processes, including color processes, and lithography.
In embodiments, the present invention is directed to the preparation of a latex comprising (a) pre-emulsification and (b) emulsion polymerization processes and preferably a batch process comprised of (a) batch pre-emulsification and (c) batch emulsion polymerization with controlled latex properties whereby a stable emulsion of resin particles in water with desirable triboelectrical toner properties can be synthesized and wherein the colloidal properties of the resulting resin particles can be controlled in a manner that the latexes of resin particles may be aggregated and coalesced in the processes described in U.S. Pat. No. 5,403,693, and in similar processes, over a wider range of conditions and with improved consistency and reproducibility.
In U.S. Pat. No. 5,278,020, there is illustrated, for example, the preparation of latexes, by batch emulsion polymerization without utilizing a pre-emulsification step. When the pre-emulsification step is not employed the resulting latexes may not have the same properties and as a result the emulsion may not be stable. Therefore, unstable latexes with undesired properties have to be removed to prevent any adverse effect on the final toner. Since both emulsion batch to batch variability and unstability are generally not desired, there exists a need for a emulsion polymerization process wherein a stable emulsion of resin particles with reproducible properties can be synthesized and which resin particles can be selected for use in toner aggregation and coalescence processes.
Accordingly, the present invention is directed, in embodiments, to the pre-emulsification of polymerization reagents in water containing a mixture of anionic and nonionic surfactants, prior to the emulsion polymerization to ensure homogeneous concentration of reagents of the emulsion system throughout reactor, in order to produce a stable resin particles emulsion with consistent properties that are more desirable for aggregation/coalescence processes than the properties of similar latexes made without a pre-emulsification step. Pre-emulsification refers to a process, which, for example, comprises mixing comonomers, initiators, chain transfer agents in water containing a mixture of anionic and nonionic surfactants at low temperature such as for example from about 3.degree. to about 30.degree. C. for about 15 to about 60 minutes, using a mechanical stirrer with speed of from about 100 to about 400 rpm. The present invention in embodiments utilizes the above mentioned pre-emulsification step prior to the emulsion polymerization step to minimize or eliminate batch to batch variation of latex properties and minimize the production of resin particles with undesirable properties which increases the breakdown of aggregated particles that may occur when practicing the processes disclosed in U.S. Pat. No. 5,403,693 and similar processes, thereby resulting in a superior process wherein the toner properties are controlled over a substantially wider range of conditions.
While not being desired to be limited by theory, it is believed that during emulsion polymerization in the water, a hydrophobic monomer or mixture of comonomer and chain transfer agents can be formed. At the start of polymerization most of hydrophobic monomers or mixture of comonomers and chain transfer agents are located in large droplets, or monomer droplets, some of which are solubilized within the micelles (polymerization sites) and some in the water. As the polymerization reaction proceeds within the monomer-swollen micelles, these micelles deplete the monomer and establish a thermodynamic driving force which causes mass transfer from the monomer droplets through the water phase into the newly formed and growing polymer particles. When a mixture of nonpolar and polar comonomers and chain transfer agents are employed in an emulsion polymerization, it is important that the concentration of polar and nonpolar comonomers and chain transfer agents be the same at any given time in all monomer droplets and in all micelles. This ensures that all polymer particles produced in emulsion polymerization have the same properties and that the emulsion polymerization product is stable and reproducible. In the present invention, emulsification prior to emulsion polymerization (pre-emulsification) is included to ensure the concentration of polymerization ingredients are the same or similar in the monomer droplets, in water, and in the micelles or growing polymer particles, thereby eliminating the formation of particles with undesirable properties and production of unstable emulsion.
While not being desired to be limited by theory, it is believed that in the emulsion polymerization some microsuspended droplets may formed which can polymerize via suspension polymerization kinetics to form polymer particles which have different properties comparable with those formed by emulsion kinetics. Therefore, an emulsion product containing some polymer particles having undesirable properties is produced which is not suitable both for aggregation/coalescence processes and for generating toner compositions. The formation of a microsuspension can also aggravate problems associated with the exotherm or cause an increase of reaction temperature during emulsion polymerization. The use of water soluble comonomers and/or chain transfer agents in an emulsion polymerization amplify the formation of undesirable microsuspended droplet and microsuspension polymer particles in the emulsion product. When water soluble comonomers and/or chain transfer agents are used in an emulsion polymerization system, it is important to employ a pre-emulsification step prior to emulsion polymerization with optimum process conditions to eliminate formation of microsuspension.
Accordingly, the present invention is directed, in embodiments, to the pre-emulsification of polymerization reagents including water soluble comonomers and/or chain transfer agents in water containing a mixture of anionic and nonionic surfactants, prior to the emulsion polymerization to prevent formation of microsuspension polymer particles and to ensure a homogeneous concentration of reagents to produce a stable emulsion with consistent properties that are more desirable for aggregation/coalescence processes than the properties of similar latexes prepared without pre-emulsification; and minimizing problems associated with exotherm during emulsion polymerization. The pre-emulsification refers to a process, which comprises mixing comonomers, or comonomer, initiators, chain transfer agents and water, containing a mixture of anionic and nonionic surfactants at low temperature, such as for example from about 3.degree. C. to about 30.degree. C. for an effective period of time, for example for about 15 to about 60 minutes, using a mechanical stirrer operating at a speed of from about 100 to about 400 rpm. The present invention in embodiments utilizes the above mentioned pre-emulsification step prior to the final emulsion polymerization to minimize or eliminate batch to batch variation of the latex properties during emulsion polymerization and to minimize production of resin particles with undesirable properties which increases the breakdown of aggregated particles that may occur when practicing the processes disclosed in U.S. Pat. No. 5,403,693 and similar processes, thereby resulting in a superior process wherein the toner properties are controlled over a substantially wider range of conditions. As illustrated in the Examples hereinafter, pre-emulsification prior to emulsion polymerization generates a stable emulsion with a minimum exotherm and provides polymer particles with desired property reproducibly.
In embodiments, the present invention is directed to a two step pre-emulsification and emulsion polymerization process for synthesizing a latex for use in aggregation/coalescence processes for preparing toner, reference the processes disclosed in U.S. Pat. No. 5,403,693, wherein a stabilizer is added to a suspension of aggregates prior to heating the aggregates to a sufficiently high temperature to enable fusing, or coalescence of the aggregates, the action of the stabilizer being to prevent further growth of the aggregates during the coalescence stage. It is believed that the use of termination agents during preparation of the latexes by emulsion polymerization, namely, agents such as certain chain transfer agents which are believed to cause adequate termination either in the aqueous phase or at the interfaces of reacting latex particles during emulsion polymerization with polar comonomers, results in latex particles with improved colloidal properties in that the breakup of aggregates during the coalescence stage is minimized or prevented with better control and over a wider range of conditions than is often achieved otherwise.
The present invention in embodiments is directed to an in situ process comprised of (i) first dispersing a pigment, such as a cyan like SUNSPERSE CYAN.TM. or SUNSPERSE RED.TM., available from Sun Chemicals, in an aqueous mixture containing a cationic surfactant, such as benzalkonium chloride (SANIZOL B-50.TM.), utilizing a high shearing device, such as an IKA/Brinkmann Polytron, or microfluidizer or sonicator; (ii) thereafter shearing this mixture with a charged latex of resin particles, such as poly(styrene/butylacrylate/acrylic acid), synthesized using a termination agent which ensures adequate termination either in the aqueous phase or at the interfaces of emulsion particles, and wherein the resin particle size ranges from about 0.01 to about 0.5 micron as measured by the Brookhaven nanosizer in an aqueous surfactant mixture containing an anionic surfactant, such as sodium dodecylbenzene sulfonate, for example NEOGEN R.TM. or NEOGEN SC.TM., and nonionic surfactant, such as alkyl phenoxy poly(ethyleneoxy) ethanol, for example IGEPAL 897.TM. or ANTAROX 897.TM., thereby resulting in a flocculation, or heterocoagulation of the resin particles with the pigment particles; (iii) further stirring for from about 1 hour to about 24 hours with optional heating at from about 0.degree. to about 25.degree. C. below the resin Tg, which Tg is in the range of between 45.degree. C. to 90.degree. C. and preferably between about 50.degree. C. and 80.degree. C., resulting in formation of statically bound aggregates ranging in size of from about 0.5 micron to about 10 microns in volume average diameter size as measured by the Coulter Counter (Microsizer II); and (iv) adding concentrated (from about 5 percent to about 30 percent) aqueous surfactant solution containing an anionic surfactant, such as sodium dodecylbenzene sulfonate, for example NEOGEN R.TM. or NEOGEN SC.TM., or nonionic surfactant, such as alkyl phenoxy poly(ethyleneoxy) ethanol, for example IGEPAL 897.TM. or ANTAROX 897.TM., in controlled amounts to prevent any changes in particle size and in GSD of the size distribution, which can range from about 1.16 to about 1.28, during the heating step, and thereafter, heating to 10.degree. C. to 50.degree. C. above the resin Tg to provide for particle fusion or coalescence of the polymer and pigment particles; followed by washing with, for example, water to remove surfactants, and drying, whereby toner particles comprised of resin and pigment with various particle size diameters can be obtained, such as from 1 to 12 microns in average volume particle diameter, and preferably in the range of 3 to 9 microns, and wherein the stirring speed in (iii) is reduced in (iv) as illustrated in U.S. Pat. No. 5,403,693. The aforementioned toners are especially useful for the development of colored images with excellent line and solid resolution, and wherein substantially no background deposits are present. This invention is directed in embodiments to the synthesis of latex particles whereby the behavior of the suspended aggregates in the heating, or coalescence step as well as properties of the generated toners are improved.
Of importance with respect to the processes of the present invention in embodiments is employing a pre-emulsification prior to the start of emulsion polymerization which comprises the steps of (i) adding the mixture of polar and nonpolar comonomers, initiators, and a mixture of chain transfer agents including at least a water soluble chain transfer agent to water containing a mixture of an anionic and a nonionic surfactant in a pre-emulsification vessel; (ii) emulsifying the content of the emulsification vessel using a mechanical stirrer to obtain a homogeneous emulsion with substantially no microsuspension droplets; (iii) transferring the homogeneous emulsion to a reactor; (iv) polymerizing the homogeneous emulsion by heating from ambient temperature, about 25.degree. C., to about 70.degree. C. to form a stable emulsion of polymer particles with desirable properties for aggregation/coalescence processes; and (v) controlling the amount of anionic or nonionic surfactant added to already formed aggregates, as disclosed in U.S. Pat. No. 5,403,693, to ensure, for example, that the dispersion of aggregated particles remains stable and thus can be effectively utilized in the coalescence process in a manner which maintains control of particle size and particle size distribution. The addition of the added anionic or nonionic surfactant prior to coalescence increases the repulsion between the aggregates, thus enhancing the stability of the aggregated system against a further increase in aggregate size to such an extent that the aggregates can essentially retain their particle size during the coalescence step. Controlling the amount of added surfactant can be important, especially for the preparation of small toner composite particles, since one can control particle growth in the aggregation step and retain those particles through the heating stage. These advantages are disclosed in U.S. Pat. No. 5,403,693. Conversely, the aggregates may break apart into smaller entities upon addition of this extra stabilizer and subsequent heating, which is detrimental to the process and product. The tendency of the aggregates to break apart, when it occurs, may be reduced or eliminated by reducing or eliminating formation of any undesirable polymer particles during emulsion polymerization using a pre-emulsification step.
There is illustrated in U.S. Pat. No. 4,996, 127 a toner of associated particles of secondary particles comprising primary particles of a polymer having acidic or basic polar groups and a coloring agent. The polymers selected for the toners of this '127 patent can be prepared by an emulsion polymerization method, see for example columns 4 and 5 of this patent. In column 7 of this '127 patent, it is indicated that the toner can be prepared by mixing the required amount of coloring agent and optional charge additive with an emulsion of the polymer having an acidic or basic polar group obtained by emulsion polymerization. Also, note column 9, lines 50 to 55, wherein a polar monomer, such as acrylic acid, in the emulsion resin is necessary, and toner preparation is not obtained without the use, for example, of acrylic acid polar group, see Comparative Example I. Unlike in the present invention, aggregates in the process described by U.S. Pat. No. 4,996,127 continue to increase in size when the temperature of the suspension of aggregates is increased, including when the suspension is heated in order to fuse the aggregates. No method of minimizing or preventing the growth of aggregates prior to fusing, or coalescence is disclosed, nor is a method disclosed for reducing the tendency of aggregates to break apart upon addition of extra stabilizers and subsequent heating, when such a tendency arises. Furthermore, the use of a pre-emulsification step prior to emulsion polymerization to advantageously eliminate latex particles with undesirable properties is not disclosed. In U.S. Pat. No. 4,983,488, there is illustrated a process for the preparation of toners by the polymerization of a polymerizable monomer dispersed by emulsification in the presence of a colorant and/or a magnetic powder to prepare a principal resin component, and then effecting coagulation of the resulting polymerization liquid in such a manner that the particles in the liquid after coagulation have diameters suitable for a toner. It is indicated in column 9 of this patent that coagulated particles of 1 to 100, and particularly 3 to 70 are obtained. This process is thus primarily directed to the use of coagulants, such as inorganic magnesium sulfate which results in the formation of particles with wide GSD, Similarly, the aforementioned disadvantages are noted in other prior art, such as U.S. Pat. No. 4,797,339, wherein there is disclosed a process for the preparation of toners by resin emulsion polymerization, wherein similar to the '127 patent polar resins of opposite charges are selected, and wherein flocculation is not disclosed; and U.S. Pat. No. 4,558,108, wherein there is disclosed a process for the preparation of a copolymer of styrene and butadiene by specific suspension polymerization, and not emulsion polymerization. Other patents mentioned are U.S. Pat. Nos. 3,674,736; 4,137,188 and 5,066,560.
In U.S. Pat. No. 5,290,654, the disclosure of which is totally incorporated herein by reference, there is disclosed a process for the preparation of toners comprised of dispersing a polymer solution comprised of an organic solvent and a polyester, and homogenizing and heating the mixture to remove the solvent and thereby form toner composites. Additionally, there is disclosed in U.S. Pat. No. 5,278,020, the disclosure of which is totally incorporated herein by reference, a process for the preparation of in situ toners comprising a halogenization procedure which, for example, chlorinates the outer surface of the toner and results in enhanced blocking properties.
In U.S. Pat.No. 5,308,734, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for the preparation of toner compositions which comprises generating an aqueous dispersion of toner fines, ionic surfactant and nonionic surfactant, adding thereto a counterionic surfactant with a polarity opposite to that of said ionic surfactant, homogenizing and stirring sid mixture, and heating to provide for coalescence of said toner fine particles.
In U.S. Pat. No. 5,346,797, the disclosure of which is totally incorporated herein by reference, there is disclosed a processs for the preparation of toner composittions comprising
(i) preparing a pigment dispersion in a water, which dispersion is comprised of a pigment, an ionic surfactant, and optionally a charge control agent; PA1 (ii) shearing the pigment dispersion with a latex mixture comprised of a counterionic surfactant with a charge polarity of opposite sign to that of said ionic surfactant, a nonionic surfactant and resin particles, thereby causing a flocculation or heterocoagulation of the formed particles of pigment, resin and charge control agent to form electrostatically bounded toner size aggregates; and PA1 (iii) heating the statically bound aggregated particles to form said toner composition comprised of polymeric resin, pigment and optionally a charge control agent.
Emulsion/aggregation processes for the preparation of toners are illustrated in a number of patents, such as U.S. Pat. No. 5,290,654, U.S. Pat. No. 5,278,020, U.S. Pat. No. 5,308,734, U.S. Pat. No. 5,346,797, U.S. Pat. No. 5,370,963, U.S. Pat. No. 5,344,738, U.S. Pat. No. 5,403,693, U.S. Pat. No. 5,418,108, U.S. Pat. No. 5,364,729, and U.S. Pat. No. 5,346,797, the disclosures of which are totally incorporated herein by reference. In U.S. Pat. No. 5,403,693, there is illustrated an emulsion-aggregation process where during the process there is added further anionic or nonionic surfactant in the range of from about 0.1 to about 10 percent by weight of water to control, prevent, or minimize further growth or enlargement of the particles in the coalescence step. The present patent application in embodiments relates to the use of a pre-emulsification step prior to emulsion polymerization in the preparation of polymer particles by emulsion polymerization in a manner that reduces the exotherm during polymerization, minimizes formation of polymer particles with undesirable properties and minimizes batch to batch variation of emulsion product.